I got a plan of 100 Mbps download speeds, but I'm only getting 40~50 Mbps while having only 1 device (a win 10 laptop with 4 core CPU and 16GB RAM) connected. I'm using N600 WiFi Cable Modem Router (C3700).
I have chatted with an agent on Xfinity Assistant to reflect this on Sunday (1/24) and followed their instruction and rebooted my modem router, the speeds were increased a little bit to around 50 Mbps for a moment but it dropped back to around 45 again. They said they would report this to the technician team and a technician would reach out to me. Today is already Thursday (1/28) but I haven't received any call back or feedback in any form.
Are you connecting to the router with an ethernet cable, or WiFi?
Ethernet will support 10/100/1000 Mbps. Almost all of the 10's are gone by now. However, some of the older WiFi standards support less than 50 Mbps.
If you are using ethernet, you should be getting close to the 100Mbps you are subscribed to. If WiFi, more variables come into play, but I never got my laptop to connect at above 50 Mbps.
As stated above you really can't validate your internet connection speed with wifi, you need to hardwaire at least a 100mb ethernet cable temporarily, otherwize you're just testing your wifi in your laptop and that varies greatly based on hardware and distance.